Many emerging material processing applications in the semiconductor and communications fields require sub-micron processing capability. A number of competing technologies exist that either have, or will soon have, this capability, such as; electron beam etching, plasma etching, x-ray lithography, and machining with ultrafast pulse lasers (laser machining). Of these technologies, only laser machining provides the advantages of operation in a standard atmosphere and in situ monitoring.
An important feature of ultrafast pulse lasers is their capability to ablate surface regions smaller than their minimum, diffraction limited, spot size. This capability is created by the brevity of the pulse, which allows for essentially no spreading of heat during the pulse, and the Gaussian spatial beam profile. By carefully controlling the energy of a pulse, it is possible to raise the intensity of only a small region in the center of the beam above the ablation threshold for the material being machined. Because of the lack of heat conduction in the pulse duration, only the small region is ablated. In this way, holes may even be laser machined with diameters less than the wavelength of the laser, for example holes having a diameter of approximately 500 nm may be drilled using a 775 nm femtosecond pulse laser. Geometrically speaking, it is possible to space these holes as close as 500 nm. When the holes are drilled one by one from one end to the other with the same laser, however, the previous method of laser machining a series of holes, the hole center-to-center spacing (pitch) cannot approach this limit.
The following example illustrates this problem. Assume that the first hole is drilled with certain laser intensity and a certain number of laser pulses. The laser intensity is chosen so that laser-induced ablation occurs only in the central portion of beam spot formed on the surface, where the breakdown threshold is reached. This ablation leads to hole drilling. Even though the surrounding area that is irradiated does not reach ablation threshold, however, it may undergo material property changes that increase the ablation threshold for subsequent laser irradiation. This phenomenon of laser irradiation-induced material hardening, laser hardening hereinafter, means that using the same laser intensity and number of pulses on the hardened area, a new hole may not be drilled in the laser hardened region. Therefore the hole-drilling reliability and reproducibility suffers. This issue is of particular importance in a device, such as a photonic crystal, in which a large number of substantially identical holes placed with a precise sub-micron pitch are desired.
A solution to this problem is an exemplary laser machining process of the present invention, which allows closer placement of the holes to reach sub-wavelength center-to-center hole spacing (pitch).
The first step of this exemplary process is to separate the hole positions on the surface of the material sample into two groups selected so that no two members of either group have a pitch less than the laser beam spot size. Next the pulse energy of the laser beam is set to a predetermined level, selected to drill holes of the desired diameter in the surface. Then the sample is positioned so as to focus the laser beam on the surface at a hole position in the first group and a number of laser pulses are applied to ablate the surface, thereby forming a hole in the surface. The process is repeated for every hole position of the first group.
At this point the pulse energy of the laser beam is set to a second predetermined level, selected to drill holes of the desired diameter in the surface once it has been laser hardened. Then the sample is positioned so as to focus the laser beam on the surface at a hole position in the second group and a number of pulses of the laser beam are applied to ablate the surface, thereby forming a hole in the surface. Alternatively, the pulse energy of the beam may be maintained at the same level and a greater number of pulses applied to the laser hardened surface. The process is repeated for every hole position of the second group.
Alternatively, the laser beam may be moved rather than the sample.
Another aspect of the present invention is an exemplary photonic crystal comprising a plurality of holes formed in a material sample by the method described above.